The present invention relates to disc drives. More specifically, the present invention relates to improving the accuracy of a servo pattern timing reference on a disc in a disc drive.
Disc drives illustratively include data transducers located relative to disc surfaces of rotatable discs in a disc stack. The data transducers are provided with a write signal to encode data on the disc surface. When the disc surface is moved relative to the data transducer, the data transducer generates a read signal indicative of data which has already been encoded on the disc.
In order to write data to the disc, a servo system is used to position the data head at one of a plurality of concentric tracks on the disc surface. A disc drive controller then provides information which generates the write signal that is provided to the data transducer. The data transducer thus encodes data on the disc surface at the desired track location.
When a read operation is to be performed, the servo system again positions the data transducer relative to a desired track on the disc surface. The data transducer then generates a read signal indicative of information encoded on the track over which the data transducer is positioned. This information is provided back to the drive controller which identifies data based on the read signal received.
It can thus be seen that, in order to perform a read or write, the servo system must perform a track following operation. In a track following operation, the servo system holds the data transducer over a track on the disc surface, while the disc rotates, to read data from, or write data to, the disc surface. In order to access a desired portion of the disc surface, the servo system must perform a track seek operation. In the track seek operation, the servo system moves the data transducer radially relative to the disc surface to a desired one of the concentric tracks to be accessed.
In order to accomplish positioning of the data head relative to the disc track, servo information written on the disc track by an external servo writer is utilized. The servo information includes a timing reference also known as a clock track. The clock track is written to the disc using one, common, crystal-generated frequency. The clock track is used as a reference for motor speed control and for servo pattern generation. One full revolution corresponding to the clock track is determined by an index which is included on the motor controller. The clock track is, itself, written with an encoded index.
As servo pattern frequency continues to increase, and as the discs continue to rotate at higher RPMs, the frequency capability of the head/disc interface has increased as well. Thus, the timing accuracy of the timing reference system used to generate the written patterns on the disc becomes increasingly critical.
When the clock track is written in a single revolution, it is very difficult to maintain the phase of the clock signal precisely accurate such that the beginning of the clock track exactly coincides with the ending of the clock track. For example, if the clock track is simply written with a raw timing source, variations in the oscillator frequency and variations in the disc RPM can make it extremely difficult, if not impossible, to make the beginning and end of the clock track precisely coincide with no phase difference. The phase difference at the starting/ending of the clock track is referred to as the splice phase error or simply splice error.
Traditionally, phase locked oscillators (PLOs) have been used in disc drives in order to lock on to the timing reference for pattern generation, etc. At the end of the clock track, if the splice error were fed into the PLO, this can cause frequency modulation. As the timing reference is scaled up to frequencies used by the remainder of the drive, this can cause many problems.
While the PLO can be designed to address certain errors, if the PLO were designed to specifically address the splice error, this would lead to many other difficulties. For example, the PLO is commonly designed to address many considerations, such as misplaced bits, mechanical resonance at various frequencies, etc. Thus, the splice error continues to present problems.
In addition, PLO jitter can also be a problem. PLO jitter refers to the phase difference between the raw clock track signal input to the PLO and the output from the PLO. Jitter can be increased for many reasons. For example, background noise simply due to the electronics operating and switching in the system, can lead to increased jitter in the PLO as the read head travels around the clock track.
Similarly, many current disc drive manufactures have products which must operate at many different frequencies. Various different frequencies can lead to increased PLO jitter as well. In addition, the drive may be required to operate at different frequencies to check various aspects of the drive, such as bearing performance, windage, spring biases, head ringing due to suspension structural mode excitation, etc. The PLO jitter may be different at each of these frequencies.
The present invention addresses one or more of these disadvantages and offers one or more advantageous features over the prior art.
It is generally believed that, the more pure the servo pattern timing reference, the better. A system writes a servo clock track on a disc in a disc drive. The clock track is written on the disc and the servo system is configured to reduce a splice error in a timing signal based on the clock track. The system is also configured to reduce phase-locked oscillator (PLO) jitter in the timing signal.